The turn motif is a phosphorylation switch that regulates the binding of Hsp70 to protein kinase C.

Heat shock proteins play central roles in ensuring the correct folding and maturation of cellular proteins. Here we show that the heat shock protein Hsp70 has a novel role in prolonging the lifetime of activated protein kinase C. We identified Hsp70 in a screen for binding partners for the carboxyl terminus of protein kinase C. Co-immunoprecipitation experiments revealed that Hsp70 specifically binds the unphosphorylated turn motif (Thr(641) in protein kinase C beta II), one of three priming sites phosphorylated during the maturation of protein kinase C family members. The interaction of Hsp70 with protein kinase C can be abolished in vivo by co-expression of fusion proteins encoding the carboxyl terminus of protein kinase C or the carboxyl terminus of Hsp70. Pulse-chase experiments reveal that Hsp70 does not regulate the maturation of protein kinase C: the rate of processing by phosphorylation is the same in the presence or absence of disrupting constructs. Rather, Hsp70 prolongs the lifetime of mature protein kinase C; disruption of the interaction promotes the accumulation of matured and then dephosphorylated protein kinase C in the detergent-insoluble fraction of cells. Furthermore, studies with K562 cells reveal that disruption of the interaction with Hsp70 slows the protein kinase C beta II-mediated recovery of cells from PMA-induced growth arrest. Last, we show that other members of the AGC superfamily (Akt/protein kinase B and protein kinase A) also bind Hsp70 via their unphosphorylated turn motifs. Our data are consistent with a model in which Hsp70 binds the dephosphorylated carboxyl terminus of mature protein kinase C, thus stabilizing the protein and allowing re-phosphorylation of the enzyme. Disruption of this interaction prevents re-phosphorylation and targets the enzyme for down-regulation.